Process of forming an electrically insulating and anticorrosive oxide coating on aluminum material



Nov. 12, 1929. s. SETOH ET AL 1,735,509

PROCESS OF FORMING AN ELECTRICALLY INSULATING AND ANTICORROSIVE OXIDE COATING ON ALUMINIUM IATERIAL Filed Oct. 22. 1926 5. erial:

BY WM #M A TTORNEYS.

Pal:ented Nov. 12, 1929- T N ED STATES, PATENT OFFICE SH OJ'I SETOH, OF TOKYO, AND SAKAE. UEKI, DECEASED, LATE F OAZA, JAPAN, BY

UNO UEKI, OF OAZA, J'A PAN," ADMINISTRATRIX, RIKAGAKU KENKYUJ'O, OF TOKYO, 'J'AIAN ASSIGNORS T0 ZAIDAN HOJIN PROCESS OF FORMING AN ELECTRIOALLY INSULATING AND ANTICORROSIVE OXIDE COATING ON ALUMINUM MATERIAL Application filed October 22, 1926, Serial No. 143,528, and in Japan October 30, 19 25.

This invention relates to an improvement of a process offorming an electrically insulating and anti-corrosive oxide coating on aluminum material comprising electrolyzin 5 a solution having oxalic acidor oxalates, an

employing alumlnum material as the positive electrode for the direct current, and aluminum materialor other electric conductive ma- I terial as the negative electrode, characterized 1 by the fact that the alternating current is also supplied to the electrodes simultaneous with the supply of the direct'current. The obj ect of the invention is to obviate the tendency L =of the aluminum material comprising the positive electrode to corrode during the electrolytic operation when direct current only is used, as well as to increase the speed of formation of the oxide coating owing to the superposed effect of both currents. One example of an apparatus for carrying out the invention is shown in the accompanying drawing, in which a is a transformer with its secondary neutral point connected to the positive terminal of b, a source of direct current, 0 the negative electrode, 03 and d the positive electrodes and e an electrolytic bath in section. The solution may be 1.0 to 3.0%

aqueous solution of oxalic acid or oxalates such'as sodium oxalate or potassium oxalate. The negative electrode a may be of any convenient conducting material while a! and d are of aluminum material which is to be coated with an oxide coating. From the source of direct current I), a direct current of, say, 60 to 90 volts is supplied and at the same time an alternating current of 60 to 120 volts is also supplied to the two positive electrodes ti and d, the total density of the current being from 0.05 to 0.2 amperes per square centimeter.

In carrying outthe process described in U. S. patent application Serial No. 7 53,525, when alternating current only is used, there is the defect that the speed of formation of the oxide coating is small and therefore the time necessary for forming the oxide coating of desired thickness prolonged. When direct current only is used for the purpose, although the speed of formation of the oxide 50 coating is great, there is a great drawback in that the aluminum material under treatment is attacked or corroded at oints where the c rculation of electrolyte is inadequate, es eclally 1f agitation of the electrolytic solution is insuflicient, or if the temperature of the,

solution is high. This corrosive efi'ectoften leads to breakage of the aluminum material during the treatment;

Now according to this invention, there are two great merits in that, firstly, during the operation there is no danger of the aluminum material corroding; secondly, the speed of formation of the oxide coating becomes great and consequentl the time required therefor is remarkably s ortened. The reasons why such a desirable result can be obtained by this 7 junctions or contacting points, and when the temperature of the electrolytic solution is raised and therefore such special points finally become corroded and attacked. This drawback, however, is not met when alternating current only is used, because in using A. C. oxalic acid radicals which have collected on the surface during a half wave are repelled by the voltage in the opposite direction and can not thus accumulate on the electrodes. Therefore by using A. C. simultaneously with D. (1., the drawback of the aluminum ma.- terial corroding is obviated and at the same time, tlTe time required for forming a desired thickness of oxide coating is shortened because the speed of such formation becomes greater by the influence of A. C. over the action of D. (l, which, as beforesaid, has in itself comparatively large speed for the formation.

Claims:

1. The process of forming an electrically insulating and anti-corrosive oxide coating on aluminum material which comprises simultaneously electrolyzing a solution containing an oxalic acid compound with alternating and direct current employing aluminum material as the positive electrode and electric-conductin material as the negative 5 electrode for the direct current.

2. The process of forming an electrically insulating and anti-corrosive oxide coating on alummum material which comprises simultaneously electrolyzingla solution containing oxalic acid with alternating and direct current employing aluminum material as .the positive electrode and. electric conducting material as the negative electrode for the direct current.

In testimony whereofwe have afiixed our signatures.

SHOJI SETOH. UNO UEKI, Administratfim of the Late Sakae Uelcz', Deceased. 

